The present invention relates to a VSC-converter for converting direct voltage into alternating voltage and conversely and which has at least two phase legs having each at least two current valves connected in series, said valves consisting of at least a semiconductor element of turn-off type and a rectifying member connected in anti-parallel therewith, a mid point of the phase leg between said valves being adapted to form a phase output and to be connected to a phase of an alternating voltage network
Such a VSC-converter for connection between a direct voltage network and an alternating voltage network is already known through for example the thesis xe2x80x9cPWM and control of two and three level High Power Voltage Source Convertersxe2x80x9d by Anders Lindberg, Royal Institute of Technology, Stockholm, 1995 in which publication a plant for transmitting electric power through a direct voltage network for high voltage direct current (HVDC) while utilizing such a converter is described. Before the creation of this thesis plants for transmitting electric power between a direct voltage network and an alternating voltage network have been based upon the use of network commutated CSC(Current Source Converter)-converters in stations for power transmission. However, in this thesis a totally new concept is described, which is based on instead using VSC(Voltage Source Converter)-converters for forced commutation for transmitting electric power between a direct voltage network being voltage stiff therethrough, in the case in question for high voltage direct current, and alternating voltage networks connected thereto, which offers several considerable advantages with respect to the use of network commutated CSC-converters in HVDC, among which it may be mentioned that the consumption of active and reactive power may be controlled independently of each other and there is no risk for commutation faults in the converter and thereby no risk for transmitting commutation faults between different HVDC-links, which may take place for network commutated CSC-s. Furthermore, there is a possibility to feed a weak alternating voltage network or a network without any generation of its own (a dead alternating voltage network). There are also further advantages.
The invention is not restricted to this application, but the converter may just as well be intended for conversion in an SVC in which the direct voltage network is then replaced by a DC-intermediate link. xe2x80x9cNetworkxe2x80x9d is also to be given a very broad sense, and it has not to be a question about any such networks in the normal meaning of this word. The voltages of the direct voltage side of the converter are advantageously high, 10-400 kV, preferably 50-400 kV.
When transmitting direct voltage on a direct voltage network connected to the converter it is desired to have a voltage being as high as possible, since the transmission losses are reduced when the voltage increases. However, an increase of this voltage means that the number of semiconductor elements of turnoff type connected in series in the current valves of the converter has to be higher so as to together be able to take the voltage required, which is distributed among them. This means that the cost for the high number of semiconductor elements of turn-off type and rectifying members (rectifying diodes) will be high. This cost is particularly unfavourable when transmitting low powers on the direct voltage network, since the fact that the power is proportional to the voltage will mean a much lower voltage across the converter than the voltage it is dimensioned for, so that this and also the conductor of the direct voltage network, which is designed for higher voltages, are poorly utilized. Thus, the cost per volt becomes in such a case very high.
The object of the present invention is to provide a VSC-converter of the type defined in the introduction, which is more cost efficient than the converters already known with respect to the ability to generate certain voltage levels on the direct voltage side of the converter.
This object is according to the invention obtained by connecting the phase legs of such a converter in series, and the opposite ends of the series connection formed by an outer end of the respective outer phase leg in the series connection are intended to be connected to a pole conductor each of a direct voltage network.
By connecting the phase legs of the converter in series in this way between the two pole conductors of the direct voltage network each current valve will at a determined voltage of the direct voltage side have a lower voltage to hold in the blocking state thereof than in VSC-converters already known, where the different phase legs are connected in parallel between the two pole conductors of the direct voltage side, so that a lower number of semiconductor elements of turn-off type and rectifying members connected in series or such ones dimensioned for lower voltages and thereby being less expensive may be used in each current valve for obtaining the voltage in question. Another way to express this is to say that for a given set of semiconductor elements of turn-off type and rectifying members of the converter and thereby a given cost thereof a higher voltage may be obtained on the direct voltage side than for a VSC-converter already known through the design of the converter according to the invention, so that the cost per volt may be reduced. More exactly, a cost so defined per volt will be at the most xc2xd (in the case of a converter having only two-phase legs) of the cost of a converter of this type already known. However, in the practice three phases are mostly used which then means a reduction of the cost per volt with respect to semiconductor elements of turn-off type and rectifying members to a third, and it is also especially advantageous to connect exactly three phase legs in series in this way, since the mutual phase displacement of 120xc2x0 of the voltage therebetween results in a uniform power flow through the converter at the series connection of the phase legs.
According to a preferred embodiment of the invention a transformer is connected to each phase output between this and the alternating voltage network phase belonging thereto. A transformer normally already there for adaption of the voltage between the alternating voltage network and the direct voltage network may in this way be utilized for realizing said series connection. One winding of the respective transformer is then preferably connected through a first end to the phase output of the phase leg and through a second end in that way to the current valve that a closed loop through the current valve is formed.
According to another preferred embodiment of the invention one winding of the respective transformer is connected through a first end to the phase output of the phase leg and through a second end to a midpoint between two capacitors connected in series in parallel with the current valves of the phase leg. The invention may in this way easily be realized, in which each phase leg then preferably has two current valves connected in series, as in a conventional 2-pulse bridge. The return current to the alternating voltage network will in this case go through the two capacitors.
According to another preferred embodiment of the invention each phase leg has a NPC-connection, i.e. four current valves connected in series, in which one point of the phase leg between the two inner valves of the series connection forms said phase output, and a series connection of two so called clamping rectifying members directed in the same direction with respect to said series connection as said rectifying members is connected between on one hand a point between one outer valve in the series connection and the inner valve next thereto and on the other a point between the other outer valve of the series connection and the inner valve next thereto with a midpoint between the two clamping rectifying members connected to a zero potential defined by capacitors connected in series with the phase leg. The advantage by using a so called NPC-(Neutral Point Clamped)-connection with respect to a so called 2-pulse bridge is that a lower switching frequency may be used, which means lower losses and a higher efficiency. The number of semiconductor elements of turn-off type and rectifying members connected in series in each phase leg will for the rest be just as high as in the case of 2-pulse bridges connected in series, but each phase leg has four current valves connected in series, besides the addition of the clamping diodes.
According to another preferred embodiment of the invention the clamping rectifying members are formed by a rectifying diode and a semiconductor element of turn-off type connected in anti-parallel therewith. Such a connection of a so called NPC-converter is described in the Swedish patent application No. 9800205-8 of the applicant and the advantages thereof are thoroughly penetrated therein and involves primarily a saving of the number of semiconductor elements of the converter, since not all of them have to be controlled with high frequencies and thanks to this high voltage semiconductor elements not able to take such high frequencies without unacceptably high switching losses may be used for certain semiconductor elements, so that a considerable saving of costs and simplifying of the control of the converter may be achieved.
According to another preferred embodiment of the invention said members taking a voltage are formed by a flying capacitor connected between said points. By using flying capacitors the advantages present in a so called NPC-connection may be obtained without using any further semiconductor elements (clamping rectifying members). An advantage of the use of so called flying capacitors for obtaining additional voltage levels for the phase output besides the voltage level of the two ends of the phase leg with respect to a use of so called clamping diodes is primarily that the semiconductor elements in the latter case have to be controlled in such a way that a non-uniform distribution of switching losses among them takes place, so that in the practice all semiconductor elements have to be dimensioned for managing the maximum load to which an individual semiconductor element may be subjected, since otherwise particular considerations have to be taken to the design of each individual semiconductor element when controlling them. This makes the total cost for the semiconductor elements comparatively high, since some of them in most operation situations will be over-dimensioned. By using flying capacitors instead, such as in the device defined in the introduction, a multi level converter with a possibility to a more uniform load on the semiconductor elements with respect to switching losses may be obtained without using expensive so called clamping diodes or additional semiconductor elements.
According to another preferred embodiment of the invention each phase leg is formed by series connections of each two current valves, said series connections being connected in parallel, i.e. a so called H-bridge. An advantage of really replacing a phase leg by two phase legs or a H-bridge in such a way is that the ability to withstand current of the converter is in principle doubled, so that it is possible to take out twice as much current from the converter and thereby a higher power may be transmitted. Preferably, a winding of the transformer is for each phase leg connected to the converter with a first end connected to a midpoint between the two current valves of one series connection of current valves and the other end connected to the midpoint between the two current valves of the other series connection of current valves. An additional advantage of this converter configuration is that the voltage over the transformer winding now may assume three levels, for what reason the control of the semiconductor elements does not have to take place with the same high frequency, so that the switching losses may be kept down, which enables a higher efficiency and/or possibility to obtain an even higher power.
According to another preferred embodiment of the invention one of said pole conductors is intended to be on high voltage potential, while the other pole conductor is adapted to be on a potential close to earth for so called monopolar operation of the direct voltage network. The possible voltage for a given number of components of the current valves may hereby be increased even more, namely be doubled with respect to the case of bipolar operation with two pole conductors having a potential of the same amount but opposite signs. This means that a low voltage, low cost cable may be used as one of the pole conductors for the return current close to earth potential, but return through earth or water is also conceivable.
According to another preferred embodiment of the invention the two pole conductors are intended to be on high voltage potential of the same amount but opposite sides for so called bipolar operation. The advantage of such an arrangement is that in the case that a bipolar transmission is desired the valve functions will be fewer, the capacitors will be fewer and the transformer windings will be fewer in this way.
According to another preferred embodiment of the invention at least one inductor and a highpass filter are arranged between the respective current valve and the windings of the transformer for reducing harmonics out towards the alternating voltage network and reducing stresses on the transformer. The transformer windings will mainly thanks to this see a sinusoidal voltage between the outputs thereof with a superposed direct voltage to earth, which gives a robust solution and a less costly transformer.
According to another preferred embodiment of the invention said semiconductor elements of turn-off type are constituted by bipolar transistors with an insulated gate (IGBT=Insulated Gate Bipolar Transistor). It is particularly advantageous to use IGBT""s as semiconductor elements of turn-off type in this context, since they may easily be controlled simultaneously, so that they will behave as one single semiconductor element of turn-off type or breaker.
Further advantages as well as advantageous features of the invention appear from the following description and the other dependent claims.